The present invention relates to an improved process and apparatus for the causticisation of Bayer liquors in an alumina refinery and relates particularly, though not exclusively, to a process in which the achievable C/S ratio is significantly increased and/or in which substantially improved lime utilisation efficiencies and/or reduced alumina losses can be achieved.
In the Bayer process for alumina production, a concentrated sodium aluminate solution is produced by grinding and digesting bauxite in a caustic solution, usually under conditions of elevated temperature and pressure. After clarification of the slurry, the concentrated sodium aluminate solution is cooled and seeded with gibbsite crystals, causing gibbsite to crystallise from solution. The gibbsite is calcined to produce alunina, while the depleted (or xe2x80x9cspentxe2x80x9d) liquor is recycled to digest more bauxite.
During digestion, some of the caustic is consumed in undesirable reactions with impurities within the bauxite, reducing the liquor""s productivity. One of the most significant of these reactions results in the formation of sodium carbonate, arising from the dissolution of inorganic carbonates within the mineral phases present, or from the thermal and oxidative degradation reactions of organic compounds. Unless controlled, with each cycle of the liquor through the process the sodium carbonate concentration would continue to rise, with a corresponding reduction in the liquor""s ability to digest gibbsite or boehmite from the bauxite.
The most common technique for controlling the sodium carbonate concentration in Bayer process liquors is to causticise using either quicklime or slaked lime. This process can be carried out either within the digestion circuit itself (by introducing lime with the bauxite), or more commonly, as a side-stream process. The addition of lime directly with bauxite is not common except where lime is required to control other impurities (such as titanium or phosphorus), because the very concentrated liquors contribute to poor efficiency. Unless the temperature is very high, most of the lime undergoes side-reactions with the aluminate in solution to yield calcium aluminate species, particularly tricalcium aluminate (TCA, often also referred to as C3A in the cement industry).
In the more prevalent side-stream causticisation, a dilute liquor stream (usually taken from one of the mud washing stages) is reacted with a slaked lime slurry, generally at close to the atmospheric boiling point of the combined liquor. Alternatively, the slurry is sometimes added directly to the mud washer. The amount of sodium carbonate converted and the efficiency of lime utilisation are dependent upon many variables, but in most refineries, the lime efficiency is in the vicinity of 50 to 70%.
In the alumina industry it is common to refer to a Bayer liquor""s carbonate impurity level in terms of the caustic to soda ratio, or xe2x80x98C/Sxe2x80x99. Here, xe2x80x98Cxe2x80x99 refers to the sum of the concentrations of sodium aluminate and sodium hydroxide, expressed as the equivalent concentration of sodium carbonate. The xe2x80x98Sxe2x80x99 concentration refers to the sum of xe2x80x98Cxe2x80x99 and the actual sodium carbonate concentration, this sum once again being expressed as the equivalent concentration of sodium carbonate. It can be seen from this that a fully causticised (carbonate-free) Bayer process liquor will possess a C/S ratio of 1.00. Typically, the C/S ratio of the concentrated liquor stream in many alumina refineries is in the range 0.8 to 0.85. C/S ratios higher than this are difficult to achieve, because causticisation processes in current use are incapable of fully removing all of the sodium carbonate in the liquor streams fed to them. For example, a liquor with an S concentration of 135 g/L will typically only causticise to a C/S ratio of about 0.890. This limitation arises because the traditional implementation of the causticisation reaction with slaked lime is controlled by a number of complex equilibria, including a competing reaction involving the aluminate ion in which TCA is formed.
By contrast, the causticisation reaction of pure mixed solutions of sodium carbonate and sodium hydroxide with slaked lime is quite simple. The final concentration of hydroxide and carbonate ions is a function of the activities of the various ionic species present, in equilibrium with the solid phases calcium hydroxide and calcium carbonate. The reaction can be described by the following equation:
Ca(OH)2+Na2CO3CaCO3+2NaOHxe2x80x83xe2x80x83(1)
It has generally been assumed that the above reaction also applies when causticisation is performed in Bayer process liquors. However, it has been known for some time that calcium hydroxide reacts readily with the aluminate ion, ostensibly to form TCA. The formation of TCA is commonly held to occur via one or both of two mechanisms: a simultaneous competitive reaction in which the calcium hydroxide reacts directly with the aluminate ion to form TCA [Chaplin,. N. T., Light Metals (1971), 47-61], or a xe2x80x9creversionxe2x80x9d reaction in which the calcium carbonate formed during causticisation reacts with the aluminate. However, some authors have suggested that in Bayer liquors causticisation occurs via a xe2x80x9chydrated tricalcium aluminate intermediatexe2x80x9d [Young, R. C., Light Metals (1982), 97-117] or a xe2x80x9ccarboaluminatexe2x80x9d phase [Lectard, A; Travaux ICSOBA, 12(17), (1982), 149-156] and that TCA forms as this material ages.
Irrespective of the mechanism proposed, causticisation as practised in the Bayer process has been inefficient in terms of the C/S achieved, and in the efficiency of lime use. Furthermore, poor efficiency of lime utilisation has also meant that quite considerable amounts of aluminate ions are consumed in the formation of TCA. This can represent a substantial loss of alumina production.
A number of causticisation processes have been proposed over the years aimed at improved lime efficiency. However, these processes are generally of limited value in that they are restricted to low xe2x80x98Sxe2x80x99 concentration wash liquors, requiring large flows to be processed if sufficient mass of sodium carbonate is to be converted to compensate for the carbonate input to the refinery. In U.S. Pat. No. 2,992,893 a process is disclosed in which the thickened mud from a final mud washing stage was causticised, and then reacted with supplementary sodium carbonate to recover some of the alumina lost in the formation of TCA. The causticised liquor was then used in the mud washing stages. Apart from the xe2x80x98Sxe2x80x99 concentration limitation, this process is not ideal in that a substantial proportion of the causticised liquor is lost with the red mud residue.
An improvement over this process is described in U.S. Pat. No. 3,120,996 in which causticisation is performed in a first stage washer, supplemented by further lime addition to a third stage washer. Higher lime efficiencies were achieved (approximately 85 to 95%), but only in quite dilute washer streams (80 g/L xe2x80x98Sxe2x80x99), and the achievable C/S ratio of the causticised liquor was not very high.
A later development disclosed in U.S. Pat. No. 3,210,155 involves the direct slaking of quicklime in a clarified wash liquor that had been heated to 100xc2x0 C. After reaction, the slurry was then mixed with further wash liquor to encourage the reaction of TCA with sodium carbonate, and so recover alumina. While high C/S ratios were claimed with this process, it was restricted to wash streams with xe2x80x98Sxe2x80x99 concentrations of approximately 15 to 40 g/L.
Another process was developed in Hungary in the 1980s by Baksa et al as disclosed in U.S. Pat. No. 4,486,393. In this process, a red mud slurry from one of the washing stages was heated and fed to a reaction vessel with excess lime slurry. Apart from the xe2x80x9cnormalxe2x80x9d causticisation afforded in this tank, the excess lime reacted with sodalite and cancrinite desilication products to form a calcium hydrogamet, releasing sodium hydroxide. The discharge from this vessel was then fed to a second vessel, and further reacted with a sodium carbonate solution. This solution was obtained by salting out sodium carbonate from concentrated solutions elsewhere in the plant. The reaction of sodium carbonate with either the hydrogamet or xe2x80x9chydratedxe2x80x9d calcium aluminate resulted in the recovery of alumina and some caustic, although this step tended to reverse the gains made by formation of the hydrogamet species. While an improvement over the basic causticisation principle, lime and alumina losses through the formation of TCA are still substantial, and the achieved C/S is still limited by the carbonate/hydroxide equilibrium reaction. Furthermore, efficiency deteriorates badly if low xe2x80x98Sxe2x80x99 concentration washer streams are not utilised.
In summary, it can be seen that the prior art causticisation methods suffer from deficiencies both in the extent to which Bayer process liquors can be causticised (i.e. the maximum C/S that can be achieved), and the efficiency with which lime is utilised to effect this causticisation. By virtue of their poor lime utilisation efficiency, these processes result in the loss of aluminate from solution, thereby reducing the alumina refinery""s productivity. Further, the prior art methods are limited with respect to the concentration of the solutions that can be causticised, becoming very inefficient with liquors approaching typical first stage mud washing liquors, or mud settler overflow liquors.
The present invention was developed with a view to providing a process and apparatus for improved causticisation of Bayer liquors which is less susceptible to some of the disadvantages of the prior art noted above.
According to one aspect of the present invention there is provided an improved process for the causticisation of Bayer liquors in an alumina refinery, the process including the steps of:
reacting lime with aluminate ions in a Bayer liquor under controlled conditions of low to moderate temperature to form substantially only a hydrocalumite species and hydroxyl ions; and,
heating said hydrocalumite species in contact with a Bayer liquor under controlled conditions so as to cause the hydrocalumite species to react with the liquor to form calcium carbonate, aluminate ions and hydroxyl ions, whereby a causticised Bayer liquor is obtained and wherein the efficiency of lime utilisation is substantially increased and alumina losses minimised.
Typically the first reaction involving the formation of a hydrocalumite slurry is preformed at temperatures between about 25xc2x0 C. and 100xc2x0 C. Preferably, best performance with most Bayer liquors is obtained if the temperature is maintained between about 70xc2x0 C. and 80xc2x0 C. Preferably the first reaction occurs in a Bayer liquor which is subject to agitation.
Preferably the second reaction involving the heating of the hydrocalumite species is performed at temperatures between about 100xc2x0 C. and 180xc2x0 C. Most preferably the second reaction is performed under conditions of low shear at about 120xc2x0 C.
Advantageously the process further includes the step of adding a suitable inhibitor to the Bayer liquor at a suitable point prior to heating the hydrocalumite species whereby undesirable reaction of the hydrocalumite species to form TCA is inhibited. Preferably said inhibitor is a complexing agent and/or surfactant which is capable of being adsorbed at active sites on the surface of the hydrocalumite species to restrict the diffusion of active species at these sites. Examples of suitable surfactants include sugars such as sucrose and glucose, and polysaccharides such as starch. Most preferably anionic organic surfactants are employed. Examples of anionic organic surfactants includes the following materials, their salts, and derivatives: any anionic homopolymers or copolymers (e.g. polyacrylic acid and its co-polymers with acrylamide, or polymers bearing hydroxamate functional groups), hydroxamic acids, humic and tannic acids, lignosulphonates, fatty acids, sulphonated carboxylic acids, carboxylic acids, and polyhydroxy carboxylic acids.
Advantageously the Bayer liquor employed in the first reaction involving the formation of the hydrocalumite species has been pre-causticised whereby the C/S ratio of the pre-causticised liquor can also be further increased.
Preferably the first reaction is performed in a Bayer liquor with a moderately high A/C ratio and low free caustic. A suitable liquor will typically have an xe2x80x9cSxe2x80x9d concentration of between 40 and 350 g/L, and an A/C ratio of between 0.2 and 0.95. More preferably the liquor will have an xe2x80x9cSxe2x80x9d concentration of between 120 and 160 g/L, and an A/C ratio greater than 0.55. Typical residence time required for the completion of the first reaction is between 5 and 30 minutes, in the presence of a suitable inhibitor.
Advantageously, the hydrocalumite slurry formed in the first reaction is subject to solid/liquid separation and the hydrocalumite solids reacted with a fresh liquor to be causticised via said second reaction.
According to a still further aspect of the present invention there is provided an improved process for the causticisation of Bayer liquors in an alumina refinery, the process including the steps of:
adding a suitable inhibitor to a Bayer liquor to stabilize the formation of a hydrocalumite species during causticisation to inhibit undesirable reaction of the hydrocalumite species to form TCA, whereby the attainable C/S ratio of the liquor can be increased.
Preferably said inhibitor is a complexing agent and/or surfactant which is capable of being adsorbed at active sites on the surface of the hydrocalumite species to inhibit the diffusion of active species at these sites. Examples of suitable surfactants include sugars such as sucrose and glucose, and polysaccharides such as starch. Most preferably anionic organic surfactants are employed. Examples of anionic organic surfactants includes the following materials, their salts, and derivatives: any anionic homopolymers or copolymers (e.g. polyacrylic acid and its co-polymers with acrylamide, or polymers bearing hydroxamate functional groups), hydroxamic acids, humic and tannic acids, lignosulphonates, fatty acids, sulphonated carboxylic acids, carboxylic acids, and polyhydroxy carboxylic acids.
Preferably the improved process further comprises the step of heating the liquor during causticisation to temperatures within the range 100xc2x0 C. to 180xc2x0 C. More preferably the liquor is heated to between 120xc2x0 C. and 140xc2x0 C.
According to a further aspect of the present invention there is provided an improved process for the causticisation of Bayer liquors in an alumina refinery, the process including the steps of:
obtaining a pre-causticised Bayer liquor; and,
reacting lime with aluminate ions in said pre-causticised Bayer liquor under controlled conditions of low to moderate temperature to form substantially only a hydrocalumite species and hydroxyl ions whereby the C/S ratio of the pre-causticised liquor can be further increased.
According to another aspect of the present invention there is provided an apparatus for the improved causticisation of Bayer liquors in an alumina refinery, the apparatus including:
a conventional reactor for causticising a Bayer liquor; and
a trim reactor for reacting lime with aluminate ions in the causticised Bayer liquor under controlled conditions of low to moderate temperature to form substantially only a hydrocalumite species and hydroxyl ions whereby the C/S ratio of the causticised liquor can be further increased.
According to a still further aspect of the present invention there is provided an apparatus for the improved causticisation of Bayer liquors in an alumina refinery, the apparatus including:
a primary reactor for reacting lime with aluminate ions in a Bayer liquor under controlled conditions of low to moderate temperature to form substantially only a hydrocalumite species and hydroxyl ions; and
a secondary reactor wherein said hydrocalumite species have been subject to heating in contact with a Bayer liquor under controlled conditions so as to cause the hydrocalumite species to react with the liquor to form calcium carbonate, aluminate ions and hydroxyl ions, whereby a causticised Bayer liquor is obtained and wherein the efficiency of lime utilisation is substantially increased and/or alumina losses are minimised.
Typically said primary reactor is a stirred tank reactor in which adequate mixing of the lime and the Bayer liquor occurs to promote the first reaction.
Typically said secondary reactor is a stirred tank reactor. Alternately a pressurised tube reactor may be employed.
Preferably the apparatus further comprises means for separating the solid hydrocalumite species and the liquor from the primary reactor before reacting the hydrocalumite species in the secondary reactor with a fresh liquor.
Most preferably the liquor causticised in the secondary reactor is used as the feed liquor for the primary reactor, whereby the C/S ratio of the causticised liquor can also he substantially increased.